1. Field of the Invention
The present invention relates to a signal connecting method and apparatus for connecting waveform signals to create a synthesized waveform signal, and more particularly to a method and apparatus suitable for connecting a plurality of voice waveform signals.
2. Description of the Related Art
Voices synthesized by voice synthesizing technology are used widely nowadays. For example, voice synthesizing technology is used in various situations such as text reading software, telephone number guide, stock guide, traveller's guide, shop guide, and traffic information.
Voice synthesizing methods are classified mainly into a rule synthesizing method and a form editing method.
The rule synthesizing method performs morpheme analysis of a text from which voices are synthesized, and in accordance with the analysis results, performs a phonological process for the text to create voices. This rule synthesizing method has less constraints of the contents of a text from which voices are synthesized and can be used for voice synthesis of texts having a variety of contents. However, with the rule synthesizing method, the quality of output voices is inferior to that of the form editing method.
The form editing method records voices actually spoken by a person and coupling constituent elements obtained by dividing the recorded voices to create target voices. The form editing method is superior to the rule synthesizing method in terms of the voice quality. However, with this form editing method, it is not possible to synthesize voices which contain constituent elements unable to be derived from the recorded voices. Therefore, the larger the division unit of recorded voices, the more the constrains of voices to be synthesized. In this connection, a method capable of synthesizing voices of various types has been proposed by using the form editing method by finely dividing recorded voices to the level of vowel and consonant.
However, the waveform at the connection portion of constituent elements of recorded voices becomes discontinuous as shown in FIG. 6(a), resulting in the generation source of noises. If the division unit of recorded voices is small, noises become conspicuous because the connection portions are discontinuous and the quality of synthesized voices is lowered.
As one method of reducing such noises, it is considered, for example, to replace a discontinuous portion with a straight line as shown in FIG. 6(b) to reduce noises. However, this connection portion creates higher harmonics, also resulting in noises.
Another approach to reduce noises to be caused by discontinuous connection portions is a Minimum Distance Search (MDS) method. With this method, as shown in FIG. 6(c) when two waveforms are connected, a point having generally the same instantaneous value and tangent gradient is searched from a portion as near to the trailing edge of the forward waveform as possible and from a portion as near to the leading edge of the backward waveform, and these two points are connected together.
With the MDS method, however, the connection point of the two waveforms is generally a point different from the edge of each waveform. Parts of the waveforms to be connected are usually discarded so that synthesized waveforms become unnatural.